1. Field of the Invention
The present invention relates generally to optical amplifiers, and more particularly to optical amplifiers that cover multiple wavelength bands.
2. Description of Related Art
The demand for bandwidth continues to grow exponentially on fiber-optic superhighways due to applications such as data communications and the Internet. Consequently, there is much effort at exploiting the bandwidth of optical fibers by using higher speeds per channelxe2x80x94so-called time-division multiplexed systemsxe2x80x94and multiple wavelengths of lightxe2x80x94so-called wavelength-division multiplexing (WDM).
Most of the fiber-optic networks currently deployed use one of two kinds of fiber: standard single-mode fiber (xe2x80x9cstandardxe2x80x9d fiber) or dispersion-shifted fiber (DSF). Standard fiber has a zero dispersion wavelength around 1310 mn, and the dispersion is primarily resulting from the inherent glass dispersion. Most of the terrestrial network in the US and most of the world is, in fact, based on standard fiber. In DSF, on the other hand, waveguide dispersion is used to shift the zero dispersion wavelength to longer wavelengths. A conventional DSF will have a zero dispersion wavelength at 1550 nm, coinciding with the minimum loss in a fused silica fiber. However, the zero dispersion wavelength can be shifted around by varying the amount of waveguide dispersion added. DSF is used exclusively in two countries, Japan and Italy, as well as in new long-haul links.
The limiting factors for a fiber-optic transmission line include loss, dispersion and gain equalization. Loss refers to the fact that the signal attenuates as it travels in a fiber due to intrinsic scattering, absorption and other extrinsic effects such as defects. Optical amplifiers, for example, can be used to compensate for the loss. Dispersion means that different frequencies of light travel at different speeds, and it comes from both the material properties and waveguiding effects. When using multi-wavelength systems and due to the nonuniformity of the gain with frequency, gain equalization is required to even out the gain over the different wavelength channels.
The typical solution to overcoming these limitations is to place periodically in a transmission system elements to compensate for each of these problems. For example, FIG. 1 shows that a dispersion compensator 40 can be used to cancel the dispersion, an optical amplifier 50 can be used to balance the loss, and a gain equalization element 60 can be used to flatten the gain. Examples of dispersion compensators include chirped fiber gratings and dispersion compensating fiber (DCF). Examples of optical amplifiers include erbium-doped fiber amplifiers (EDFAs), Raman amplifiers, and non-linear fiber amplifiers (NLFAs). U.S. Pat. No. 5,778,014 discloses Sagnac Raman amplifiers and cascade lasers.
Finally, examples of gain equalizers include Mach-Zehnder interferometers and long period gratings. Rather than building a system out of these individual components, it may be easier and more cost effective to combine two or more of the functions in FIG. 1 into a single component as shown in U.S. Pat. No. 5,887,093.
Another problem that arises in WDM systems is interaction or cross-talk between channels through nonlinearities in the fiber. In particular, four-wave mixing (4WM) causes exchange of energy between different wavelength channels, but 4WM only phase matches near the zero dispersion wavelength. Consequently, if a fiber link is made from conventional DSF, it is difficult to operate a WDM system from around 1540-1560 nm. This turns out to be quite unfortunate because typical EDFA""s have gain from 1535-1565 nm, and the more uniform gain band is near 1540-1560 nm. A second fiber nonlinearity that can be troublesome is modulation instability (MI), which is 4WM where the fiber""s nonlinear index-of-refraction helps to phase match. However, MI only phase matches when the dispersion is positive or in the so-called soliton regime. Therefore, MI can be avoided by operating at wavelengths shorter than the zero dispersion wavelength.
As the bandwidth utilization over individual fibers increases, the number of bands used for transmission increases. For WDM systems using a number of bands, additional complexities arise due to interaction between and amplification in multi-band scenarios. In particular, particular system designs are needed for Raman amplification in multi-band transmission systems. First, a new nonlinearity penalty arises from the gain tilt from the Raman effect between channels. This arises because long wavelength channels tend to rob energy from the short wavelength channels. Therefore, a means of minimizing the gain tilt on existing channels with the addition of new WDM channels is required.
To minimize both the effects of 4WM and Raman gain tilt, another technical strategy is to use distributed Raman amplification. In a WDM system with multi-bands, a complexity arises from interaction between the different pumps along the transmission line.
There is a need for broadband amplifiers that span more than 40 nm of bandwidth. There is a further need for broadband amplifiers that amplify wavelengths in the low loss window of optical fibers. There is yet a further need for broadband amplifiers that reduce the cost per wavelength. There is a need for broadband amplifiers without band splitters and combiners and guard bands between the bands.
Accordingly, an object of the present invention is to provide broadband amplifiers that span more than 40 nm of bandwidth.
Another object of the present invention is to provide broadband amplifiers that amplify wavelengths in the low loss window of optical fibers.
Yet another object of the present invention is to provide a Raman amplifier apparatus with first and second Raman gain fibers.
A further object of the present invention is to provide a Raman amplifier apparatus with first and second Raman gain fibers, with optical signals traveling in a first direction and first and second pump wavelengths traveling in a reverse direction relative to the first direction.
Another object of the present invention is to provide a Raman amplifier apparatus with first and second Raman gain fibers, where a majority of longer signal wavelengths are amplified before shorter signal wavelengths.
These and other objects of the present invention are achieved in a Raman amplifier apparatus with an optical transmission line including an input to receive an optical signal, an output that passes the optical signal, a first Raman gain fiber and a second Raman gain fiber. A first WDM is positioned between the second Raman gain fiber and the output. A first set of pump wavelengths is input to the first WDM. A second WDM is positioned between the first and second Raman gain fibers. A second set of pump wavelengths is input to the second WDM. At least a portion of the first set of pump wavelengths are different than the second set of pump wavelengths. The first and second set of pump wavelengths propagate in the same direction.
In another embodiment of the present invention, an optical amplifier includes an optical fiber with an signal input port, an optical signal output port, at least a first Raman fiber amplifier and a second Raman fiber amplifier. The optical fiber is configured to be coupled to at least one optical signal source that produces an optical signal. A first WDM is positioned between the second Raman gain fiber and the output port. The first WDM is configured to be coupled to a first pump source that produces a first set of pump wavelengths. A second WDM is positioned between the first and second Raman gain fibers. The second WDM is configured to be coupled to a second pump source that produces a second set of pump wavelengths. At least a portion of the first set of pump wavelengths are different than the second set of pump wavelengths. The optical signal travels in a first direction and the first and second pump wavelengths travel in a reverse direction relative to the first direction.
In another embodiment of the present invention, an amplifier apparatus includes an optical transmission line with an input to receive an optical signal, an output that passes the optical signal, a first gain fiber and a second gain fiber. A first WDM is positioned between the second gain fiber and the output. A first set of pump wavelengths is input to the first WDM. A second WDM is positioned between the first and second gain fibers. A second set of pump wavelengths is input to the second WDM. At least a portion of the first set of pump wavelengths are different than the second set of pump wavelengths. The first and second set of pump wavelengths propagate in the same direction.
In another embodiment of the present invention, a method of amplification provides a Raman amplifier apparatus that includes an optical transmission line with an input to receive an optical signal, an output that passes the optical signal, a first Raman gain fiber and a second Raman gain fiber. A signal with multiple wavelengths is introduced into the input. The signal is amplified in the Raman amplifier apparatus. A majority of longer wavelengths of the signal are amplified before the shorter wavelengths are amplified.
In another embodiment of the present invention, a method of amplification provides an amplifier apparatus with first and second pump sources coupled to first and second ports of an optical transmission line, an output that passes the optical signal, a first gain fiber and a second gain fiber. Signals are introduced to an input of the optical transmission line. The first and second gain fibers are pumped with a first set of pump wavelengths from the first pump source and a second set of pump wavelengths from the second pump source. At least a portion of the first set of pump wavelengths is different than the second set of pump wavelengths. At least an 80 nm bandwidth of signal is amplified.